Help with volume on the three high strings while using Hammer-on's?

higher gauge strings may work, perhaps you can try only stringing the treble strings with the heavier gauge while keeping the others standard, but you'll have to ask an expert to make sure that neck warping/intonation problems won't occur. it wouldn't hurt to work on finger strength, but don't get frustrated: the higher strings are just more difficult to sound out as prominently. you can also try pulling off the higher strings with a bit of extra force so that the string is louder for the hammer on. best of luck to you